This invention relates to cathode ray tubes and, more particularly, to luminescent screens for use in such tubes.
The most common cathode ray tubes utilize a powdered phosphor on a carrier as a luminescent screen. These screens have relatively low thermal loadability since heat is insufficiently dissipated from the phosphor grains. As a consequence, during high brightness operation the phosphor has low quantum efficiency and may even be severely damaged. In addition, powdered phosphors exhibit coulomb degradation; that is, quantum efficiency declines due to electron bombardment. This problem is particularly acute in high brightness applications when high electron beam current is used (e.g., in projection CRT applications).
A partial solution to this problem is described in British patent application G.B. No. 2,000,173A which proposes that the luminescent screen be fabricated from a self-supporting monocrystalline body which includes a luminescent layer containing at least one activator. This screen purports to reduce diffuse reflections and increase heat dissipation, thus improving resolution and thermal loadability. Garnet crystal structures with Tb, Tm, Eu, Ce or Nd activators are said to be preferred.
The single crystal nature of the screen, however, gives rise to light trapping inside the monocrystalline layer which has a relatively high refractice index relative to its surroundings. This trapping phenomenon reduces the brightness which would otherwise be obtainable from the screen. However, the brightness obtainable from any luminescent screen, whether a single crystal or powdered material is used, is limited by power saturation of the phosphor; that is, beyond the saturation point, additional increases in electron beam power density do not yield significantly increased brightness. In addition, in certain cases the practical limit to achievable brightness is caused by heating of the phosphor, or by the inability to focus a high current electron beam to the desired spot size. In many applications (e.g., projection CRT), that practically achievable brightness level is insufficient.
One approach to enhancing brightness and phosphor lifetime is to make the luminescent screen (faceplate) as a series of phosphor bars, A. V. Brown et al, IBM Technical Disclosure Bulletin, Vol. 24, No. 4, pp. 2019-2020 (1981). Each bar is made to act as a light guide by suitable choice of the refractive index of the phosphor and the dielectric film supporting the bars. No reflective coating is formed on the other surfaces of the bars. A short region in the middle of the guide is created to spoil the light-guiding action of the bar, for example, by a bevel or by scattering centers. A rectangular shaped electron beam is scanned across the bars with its long side parallel to the length of the bars. Thus, a fraction of the light in each pumped bar will be guided to the center of the bar and a small light source will be created at the spoiling region in the middle. The direction of light emission, however, is primarily transverse to the length of the bar, not parallel to it. In this way, the authors claim that a very bright but small light spot is obtained from a large area of phosphor irradiated by a large area electron beam.
Unfortunately, light coupling efficiency from such a spoiling region is relatively poor which militates against the advantage of using bars pumped by rectangular e-beams. In addition, optical devices, such as microlenses, to enhance output coupling in such structures are extremely difficult to fabricate at each spoiling region.